1. Field of the Invention
The present invention generally relates to an optical path deflecting element, an optical path deflecting apparatus, an image displaying apparatus, an optical element, and a manufacturing method thereof.
2. Description of the Related Art
Conventionally, various optical elements are used as optical path deflecting elements. Among them, electrooptic devices formed of materials that have large primary electrooptic effect (Pockels effect), for instance, KH2PO4(KDP), NH4H2PO4(ADP), LiNbO3, LiTaO3, GaAs, CdTe, are well-known. Also, electrooptic devices formed of materials that have large secondary electrooptic effect, for instance, KTN, SrTiO3, CS2, nitrobenzene; are well-known. Furthermore, acoustic optical devices formed of materials, for instance, glass, silica, TeO2, are well-known (ref. “Optoelectronic Devices,” edited by S. Aoki, Shokodo). In these conventional optical elements, it is generally necessary to prepare a large length of an optical path so as to ensure a sufficiently large amount of optical deflection. Also, the conventional optical elements have limited uses due to the costly materials.
On the other hand, various optical elements formed of liquid crystal materials are conventionally proposed as the optical path deflecting elements.
Japanese Laid-Open Patent Application No. 06-018940 discloses two kinds of optical beam shifters formed of artificial birefringence plates for the purpose of reduction in optical loss of an optical space switch. One of the optical beam shifters is constituted by disposing two sheets of wedged-shaped transparent substrates to face each other and sandwiching a liquid crystal layer between the two substrates. The other is constituted by connecting the former optical beam shifter to the back surface of a matrix type deflection control element. Furthermore another optical beam shifter is proposed. In the optical beam shifter, two sheets of the wedge-shaped transparent substrates are arranged to face each other. The optical beam shifter includes a plurality of optical beam shifters capable of conducting a matrix drive between the two transparent substrates. These optical beam shifters have liquid crystal layers that can shift an incident optical beam by a half of one cell and are connected in a cascade fashion in a state where each of the optical beam shifters is shifted by a half of a cell.
Japanese Laid-Open Patent Application No. 09-133904 discloses an optical deflection switch that not only achieves large deflection and high deflection efficiency but also allows a deflection angle and a deflection distance to be arbitrarily set. In detail, two sheets of transparent substrates are disposed to face each other at a predetermined space. The facing surfaces thereof are subjected to a perpendicular orientation treatment. Ferroelectric liquid crystal of a smectic A phase is enclosed between these transparent substrates so as to have orientation perpendicular to the transparent substrates. Electrodes are arranged so that alternate current electric fields can be applied in parallel with the smectic layer. The optical deflection switch is a liquid crystal element constructed in the above configuration. In this configuration, since the optical deflection switch can use electric inclination effect caused by the ferroelectric liquid crystal of the smectic A phase, it is possible to change a refractive angle of polarized light incident to a liquid crystal layer and a displacement direction thereof by birefringence due to inclination of the liquid crystal molecules.
However, in the optical beam shifters according to Japanese Laid-Open Patent Application No. 06-018940, since nematic liquid crystal is used as the liquid crystal material therein, it is difficult to increase the response speed by a sub-microsecond. As a result, it is impossible to use the optical beam shifters in a case where high-speed switching is required.
Also, in the optical deflection switch according to Japanese Laid-Open Patent Application No. 09-133904, although the ferroelectric liquid crystal of the smectic A phase is used therein, it is impossible to expect high-speed operations because the smectic A phase has no spontaneous polarization.
Next, some prior arts related to a pixel shift element will be described.
Japanese Patent No. 2939826 discloses a projection display device for projecting an enlarged one of an image displayed in picture elements on a screen by using a projection optical system. The projection display device includes a means for shifting a projection image and a means for projecting projection areas of individual picture elements discretely on a screen through effective reduction in an aperture ratio of the picture element. The projection image shifting means has at least one optical element for rotating the polarization direction of transmitted light in an optical path between the picture element and the screen and at least one transparent element having a birefringence effect.
According to Japanese Patent No. 2939826, however, the projection image shifting means (pixel shifting means), which has at least one of the optical elements (optical rotating elements) capable of rotating the polarization direction and at least one of the transparent elements (birefringence elements) having the birefringence effect, shifts a pixel. Due to the pixel-based shifting, the projection shifting means has some disadvantages. For instance, since the projection shifting means uses a combination of the optical rotating element and the birefringence element, it is impossible to avoid large loss of the light quantity. Also, the resolution tends to be decreased by variations of a pixel shifting amount in accordance with the wavelength of the light. Additionally, a mismatch of the optical rotating element and the birefringence element with respect to the optical characteristics causes optical noise such as a ghost ray due to light leaked in an exterior of a pixel shift, that is, in the place where an image should not be formed. Additionally, it takes a heavy cost to develop and manufacture these elements in general. In particular, if the above-mentioned materials of the large primary electrooptic effect (Pockels effect), for instance, KH2PO4(KDP), NH4H2PO4(ADP), LiNbO3, LiTaO3, GaAs, CdTe, are used as the birefringence element, these disadvantages become noticeable.
Japanese Laid-Open Patent Application No. 05-313116 discloses a projector for reproducing a high-definition image at a low cost. In the projector, a control circuit samples an image, all of which should have been displayed. Sampled images are stored in an image storage circuit in accordance with a check pattern and then are supplied to a pixel selection part. Then, the sampled images are sequentially projected and displayed by using a spatial optical modulator via the pixel selection part. The control circuit controls a panel rocking mechanism corresponding to the displayed images and changes a pitch interval between adjacent picture elements of the spatial optical modulator by 1/n times (n: integer) so as to restore the image to be originally displayed through compositions of the sampled images with respect to time. According to the projector, it is possible to not only display the image under a resolving power multiplied by some integer of the picture elements of the spatial optical modulator but also implement the projector at a reasonable cost by using the pixel-coarse spatial optical modulator and a simple optical system.
According to Japanese Laid-Open Patent Application No. 05-313116, however, a pixel shift method for rocking an image display element itself by a distance smaller than the pixel pitch at a high speed is adopted. In the pixel shift method, although aberration can be suppressed due to fixing of the optical system therein, a movable part needs precision and durability because of necessity of high-accuracy and high-speed parallel-shifting of the image display element itself. As a result, other problems such as vibration and noise are caused in the projector.
Japanese Laid-Open Patent Application No. 06-324320 discloses a means for changing an optical path. The disclosed invention intends to improve apparent resolution of a displayed picture without increasing the number of picture elements-of a picture display device such as an LCD (Liquid Crystal Display). The picture display device displays a picture through light emissions of individual picture elements, which are arranged in the vertical and the horizontal directions, in accordance with a display picture element pattern. An optical member for changing an optical path for each field is provided between the picture display device and an observer or a screen. The display picture element pattern whose display position is deviated for each field in accordance with variations of the optical path is displayed on the picture display device. In this configuration, since parts whose refractive indexes differ from each other alternatively appear in the optical path between the picture display device and the observer or the screen for each field of the picture information, it is possible to change the optical path.
According to Japanese Laid-Open Patent Application No. 06-324320, a combined mechanism of an electrooptic element and a birefringence material, a lens shift mechanism, a vari-angle prism, a rotating mirror, a rotating glass and so on are described as the optical path deflecting means in the disclosure. Besides the combined mechanism of the electrooptic element and the birefringence element, an optical path switching method for switching an optical path through displacement (parallel shift or inclination) of an optical element such as a lens, a reflection plate, and a birefringence plate by using a voice coil, a piezoelectric element and so on is proposed. However, when this optical path switching method is adopted in the optical path changing means, the optical path changing means cannot help having a complicated structure for driving an optical element therein. As a result, it is impossible to avoid high fabrication cost thereof.
Japanese Laid-Open Patent Application No. 10-133135 discloses a light beam deflecting device that can operate without any rotating mechanical member. As a result, it is possible to provide the small-sized light beam deflecting device that not only has the high precision and the high resolving power but also is tolerant to external vibration. In detail, the light beam deflecting device has a translucent piezoelectric element in a propagation path of an optical beam, a transparent electrode provided on a surface of the piezoelectric element, and a voltage applying part applying a voltage to the piezoelectric element via an electrode so as to deflect the optical axis of the optical beam by changing a length of the optical path between an optical beam enter surface A and an optical beam exit surface B of the piezoelectric element.
According to Japanese Laid-Open Patent Application No. 10-133135, an optical path shifting means for shifting an optical path through variations of a thickness of the translucent piezoelectric element sandwiched between transparent electrodes by applying the voltage is disclosed. However, the optical path shifting means has problems similar to the above-mentioned optical path changing means according to Japanese Laid-Open Patent Application No. 06-324320 in that it is necessary to prepare a comparatively large transparent piezoelectric element and therefore it is impossible to avoid the high fabrication cost thereof.
Next, prior arts related to optical writing apparatuses and optical connection apparatuses will be described.
Japanese Laid-Open Patent Application No. 08-118726 discloses an optical writer for printing data at high resolution by using a focused position control means although the optical writing apparatus uses an illuminant element array of a low resolution therein. In the optical writer, the focused position control means electrooptically changes an exposed position by using a combination of a ferroelectric liquid crystal cell for rotating a polarization surface by 90° and a birefringence plate. In detail, a pair of transparent electrodes and a horizontal alignment layer are provided on a pair of transparent substrates, and a liquid crystal layer formed of ferroelectric liquid crystal of a chiral smectic A phase is sandwiched between the two substrates. When the birefringence plate is provided behind the liquid crystal cell, an optical path shifting part is constituted as follows; light propagates straight in a case where a polarization surface of the light is an ordinary ray component with respect to the birefringence plate, and, on the other hand, the light is shifted in parallel in a case where the polarization surface of the light is an extraordinary ray component. At this time, a shift amount of the light depends on an optical axis direction and a thickness of the birefringence plate. When the optical path shifting part is provided between the illuminant array and a recording body, it is possible to implement the disclosed optical writer.
According to Japanese Laid-Open Patent Application No. 08-118726, a deflection function is realized by using the combination of the ferroelectric liquid crystal cell for rotating a polarization surface by 90° and the birefringence plate. In general, an optical crystal serving as the birefringence plate is expensive. Therefore, when an optical crystal corresponding to a size of the illuminant array is used in the optical writer, it is impossible to avoid a cost increase of the optical writer.
Japanese Laid-Open Patent Application No. 07-092507 discloses an optical deflector that can deflect a plurality of incident optical beams in individual propagation directions. In the optical deflector, saw-toothed grooves are formed on a transparent substrate, and nematic liquid crystal that is aligned in a predetermined direction is enclosed between the saw-toothed substrate and a flat substrate. When the optical writer has electrodes divided in accordance with individual teeth of the saw-toothed substrate, it is possible to change a refractive index of the liquid crystal by applying a voltage to an electric field. In this fashion, the disclosed optical writer can deflect optical beams entering the teeth individually.
According to Japanese Laid-Open Patent Application No. 07-092507, since the optical writer uses the nematic liquid crystal, the optical writer has extremely insufficient response speed of several milliseconds. If the optical writer is used in an optical interconnection or the like, there arises a problem such as large propagation loss of incident light.
In summary, the disclosed conventional pixel shift elements have some disadvantages as follows.
i). Since the pixel shift elements have the complicated structures, it is impossible to avoid an increase in the size and the cost thereof and a loss of a light quantity. Furthermore, optical noise and resolution reduction are caused due to a ghost ray.
ii). If the pixel shift elements have a movable part, the pixel shift elements tend to have problems regarding position accuracy, durability, vibration and noise.
iii). When nematic liquid crystal is used in the pixel shift elements, the nematic liquid crystal has a problem regarding the response speed thereof.
Regarding the response speed in the disadvantage iii), a speed of optical deflection necessary for pixel-shifting in an image display apparatus is estimated as follows. An image field (time tField) is divided into n subfields with respect to the time. An optical path is deflected between an image display element and an optical part for each of the n subfields, and n shifted positions are determined. In this case, time of a subfield tSF is represented in the following formula;tSF=tField/n.
The optical paths are deflected during the time period tSF. Time of the optical path deflection is notated as Wshift. Since an image is not displayed in the Wshift, the optical utilization efficiency is decreased corresponding to the time period Wshift.
Here, the optical utilization efficiency E is represented as follows;E=(tSF−Wshift)/tSF.
Here, it is supposed that the pixel shift position n=4 and the image field tField=16.7 ms. In order to secure the optical utilization efficiency E of more than 90% under the supposition, it is necessary to satisfy the following inequality;0.9<(16.7/4−Wshift)/(16.7/4).
It is possible to obtain the following upper bound of Wshift from the above inequality;tshift<0.42 (ms).
Namely, it is necessary to deflect the optical path within the time period 0.42 ms. However, since common nematic liquid crystal responds at a response speed beyond several microseconds, it is impossible to use the nematic liquid crystal as a high-speed pixel shift element.
According to Japanese Laid-Open Patent Application No. 06-018940, since the nematic liquid crystal is used as the liquid crystal therein, it is difficult to improve the response speed to a sub-microsecond. Thus, the disclosed optical beam shifter cannot be adopted as the pixel shift element. Meanwhile, ferroelectric liquid crystal of a chiral smectic C phase can achieve the response speed of sufficiently less than 0.42 ms.
According to Japanese Laid-Open Patent Application No. 09-133904, the ferroelectric liquid crystal of the smectic A phase is used therein. However, since the smectic A phase does not have a spontaneous polarization, the optical deflecting switch cannot operate at high speed compared to the chiral smectic C phase.
Therefore, it is expected to propose an optical deflecting element or an optical deflector that can eliminate the above-mentioned problems, that is, high cost, a large size, light quantity loss and optical noise due to the complicated structure thereof.
Optical deflecting elements and an optical deflecting device that challenge the above-mentioned problems were presented by inventors of this specification. One of the optical deflecting elements includes a pair of transparent substrates, liquid crystal of a homogeneously-aligned chiral smectic C phase injected between the pair of substrates, and an electric field applying part formed of a pair of electrodes provided between the substrates and the liquid crystal. The optical deflecting element is provided such that an incident direction of light is different from a normal line direction of the substrate surface. Another proposed optical deflecting element includes a pair of transparent substrates, liquid crystal of a chiral smectic C phase injected between the substrates, and at least one pair of electric field applying parts. In the optical deflecting element, the two substrates are inclined to face each other corresponding to the optical deflection direction. Furthermore, the proposed optical deflecting device is formed of two pairs of such optical deflecting elements that are positioned at a predetermined interval in the light propagation direction.
According to the optical deflecting element and the optical deflecting device, since liquid crystal of the chiral smectic C phase is used as the liquid crystal material thereof, the optical deflecting element and the optical deflecting device make it possible to perform a high-speed switching operation in comparison with the above-mentioned conventional optical elements. However, it is difficult to uniformly align a region of the liquid crystal of the chiral smectic C phase. In addition, there is no conventional technique for realizing uniform alignment in a case where substrates are not arranged in parallel.
When surface-stabilized ferroelectric liquid crystal of the chiral smectic C phase, which is commonly used as a displaying element, has bistable homogeneous alignment, that is, has a stable condition with respect to two directions, it is possible to perform a successful switching operation between two bistable conditions in accordance with a polarity of an electric field applied to the displaying element. Furthermore, the surface-stabilized ferroelectric liquid crystal of the chiral smectic C phase has other advantages such as a memory property and a high-speed response property. Namely, even if the electric field becomes OFF in the stable conditions, the surface-stabilized ferroelectric liquid crystal of the chiral smectic C phase maintains the stable condition. Also, the surface stabilized ferroelectric liquid crystal of the chiral smectic C phase can respond at a higher speed than smectic A phase liquid crystal and nematic liquid crystal.
If an optical deflecting element is used to adjust a refractive angle on a slope surface through variations of a refractive index of liquid crystal molecules toward incident light by electrically switching an alignment direction of the liquid crystal molecules, it is preferable that such an optical deflecting element can vary the refractive index in a wider range so as to expand the deflection angle. Regarding characteristics of the ferroelectric liquid crystal, it is preferable that such an optical deflecting element have a large difference Δn between refractive indexes with respect to the major and the minor axes. Also, it is preferable that the smectic C phase have a large tilt angle and a large cone angle for instance, about 90°. Here, the tilt angle means a slope angle of liquid crystal molecules toward a normal line of the smectic layer, and the cone angle, which is the twice of the tilt angle, means an alignment switch angle at inversion time of an electric field. In order to accurately shift an optical path, it is necessary to provide a perfect homogeneous alignment condition to the ferroelectric liquid crystal layer injected between inclined substrates.
However, when a ferroelectric liquid crystal, whose cone angle is considerably larger than common liquid crystal, is used in order to expand the refractive index difference Δn between the molecule major and minor axes, it is extremely difficult to stably form a bistable homogeneous alignment condition. In particular, smectic liquid crystal, which is ferroelectric liquid crystal having a large cone angle, has extreme difficulty in uniform alignment because the phase transition smectic A phase is not passed.
As a result, if the large cone angle and the bistable homogeneous condition are intended, there is a probability of yield reduction in element fabrication due to defective alignment.
Additionally, when the ferroelectric liquid crystal is applied to an image displaying apparatus that adopts the above-mentioned pixel-shifting fashion, there is a probability of image degradation due to such defective alignment.